Hydraulic cylinder, piston and rod combinations are being increasingly used in fields that involve the moving and positioning of material and objects. As the various applications of hydraulic cylinder, piston and rod combinations have progressed, more stringent operational criteria are being encountered, and a need has developed to precisely, reliably and continuously sense the position of the piston and it's related displacement parameters, velocity and acceleration.
A promising approach developing in the art employs the correlation of resonant frequencies of electromagnetic waves in the cavity that is made up of the cylinder and the piston, with the dimensions of the cavity. In one aspect of this approach the wave performance is as it is in a transmission line with shorted ends in that the resonance frequency of an electromagnetic standing wave correlates inversely with the cavity length.
One application of the use of electromagnetic waves for piston position sensing is shown in U.S. Pat. No. 4,588,953 wherein the frequency, of electromagnetic waves introduced into the cylinder between the closed end of the cylinder and the piston, is swept between two limits with the frequency of the detected resonant peak being indicative of the piston position.
In U.S. Pat. No. 4,737,705, improvement is achieved with a coaxial resonant cavity with a central core within which the electromagnetic waves are launched and propagated in the mode referred to as the transverse electromagnetic wave (TEM) mode. The cylinder on the rod side of the piston is one type of coaxial cavity.
In European Patent Office (EPO) published application number 0 199 224 published Oct. 29, 1986 the detection of resonance of electromagnetic waves in a hydraulic cylinder cavity between the closed end and the piston is improved by introducing the waves in the center of the closed end of the cylinder and employing an antenna with a right angle bend extending axially from the periphery of the cylinder as the receiver in order to confine the electromagnetic wave energy launched and propagated to a mode referred to as the TM cylindrical mode.
In U.S. Pat. No. 5,182,979, compensation is provided for differences in insertion losses as the linear extension of the piston and rod in the cylinder takes place.
In U.S. Pat. No. 5,325,063 identification of a fundamental resonance frequency is enhanced by the use of positive and negative slope intercepts of a resonance signal with respect to a reference level, where the positive slope is at the lower frequency and the negative slope is at the higher frequency.
As progress in the art continues, greater precision is being sought in resonant frequency detection, and a need is developing to be able to more precisely identify a specific resonant frequency signal.